This invention relates generally to automatic switches and specifically to start-up circuits for switched-type power supplies, sometimes referred to as switch mode power supplies.
Switched-type power supplies have grown in popularity because of their low cost, and ability to deliver DC power to loads such as computer banks, with minimum internal losses. In general, a switched power supply includes a pulse width modulation (PWM) means for supplying a variable load current and a regulator for maintaining the output load voltage at a precise value. The power supply usually includes a so-called boot strap voltage which is a regulated voltage that is coupled back to the input terminal of the regulator. The power supply may be provided with a start-up circuit since the boot strap voltage is not produced until the regulator is running.
In large computer installations, for example, the power for the computer banks may be taken from 220 volt AC power line or mains and a start-up transformer is incorporated for initiating the switched power supply or supplies. Unfortunately, the input or line voltage is often subject to wide fluctuations due to the varying loads imposed upon the utility company power lines. The range of line voltages on a conventional 220 volt AC line, for example, may be from 180 volts, during a brown-out or extremely low power condition, to 250 or more volts during an over-voltage condition. Consequently, the start-up transformer, which is designed to produce a particular output voltage during low line conditions, will have an output voltage 1.4 times higher during high line conditions. The regulator in the power supply must be capable of maintaining the desired output voltage during these extremes and energy due to the excess voltage is dissipated as heat in the regulator as well as in the transformer.
One aspect of the invention solves this prior art problem by providing a start-up transformer arrangement that automatically "disconnects" from the switched power supply when the power supply boot strap voltage comes up. In so doing, the transformer is, in essence, operated without a load and the energy dissipation in the transformer under high line voltage conditions is minimal. Accordingly, the transformer may be significantly reduced in size. Also, the high line voltage is not permitted to raise the input voltage to the power supply, and therefore, the regulator in the power supply need not dissipate undue amounts of energy.
Another aspect of the invention, which is not fully utilized in the described preferred embodiment, lies in its regulating ability. Should the boot strap voltage, for some reason, fall below its normal level, the circuit of the invention will sense the lower voltage and "reconnect" the transformer and rectifier arrangement to restore and maintain the input voltage applied to the regulator at its normal level.